The invention relates to a circuit arrangement for generating the control potential for a field-effect transistor from the output voltage of a filter circuit.
A filter circuit is used for generating the control potential for a field-effect transistor if the quantity which determines the control potential is available in the form of current pulses. Deriving the control potential from these current pulses requires a filter circuit which contains a capacitance element and smoothes the current pulses by charging the capacitance element. A filter output voltage is then produced across the capacitance element and supplies the control potential for the field-effect transistor. Such a principle is employed e.g. in phase locked loops whose oscillator is controlled at the control electrode of a field-effect input transistor.
The stability of the functioning of a phase locked loop. requires that the filter circuit additionally contains a resistor as well as capacitors. The thermal noise of this resistor determines the phase noise of the oscillator if the filter circuit is designed with such a high impedance as is advantageous, in accordance with the high input impedance of the field-effect transistor, in the sense of a small area requirement in integrated circuitry. If the resistance is reduced in order to reduce the noise, then the capacitors increase in size in the same ratio and can then no longer be integrated cost-effectively.
One possible way of circumventing this problem, in particular in the case of LC oscillators, is for the control characteristic curve of the oscillator to have the flattest possible profile. Thus, for example in FM tuning circuits, it is customary to adjust the frequency by only 20% with a voltage swing of 30 volts. This requires special trimming of the oscillator, since this adjustment range does not include the production tolerances.
In integrated circuits, RC oscillators or multivibrators are customary which, on account of miniaturization, permit only operating and tuning voltages of at most 5 volts and are intended to have a wide adjustment range which, in addition to the required frequency range, is also intended to include the production tolerances as well. A large slope of the control characteristic curve of the oscillator is then unavoidable.
Phase locked loops are disclosed in the published German patent applications DE 196 34 084 A1 and DE 197 13 058 A1. In the case of the phase locked loop of DE 196 34 084 A1, a capacitive voltage divider is used for suppressing interference signals. Two diodes with mutually opposite forward directions are connected in parallel with one capacitor of the voltage divider. During on-state operation, one of the diodes bridges the capacitor, with the result that the current flowing at the output of the filter circuit is used as charge-reversal current in the ratio 1:1. The output voltage of the filter circuit is passed to a capacitive voltage divider, the tap of which carries the control potential. The fact that the phase locked loop changes its transfer properties when one of the diodes bridges the capacitor of the voltage divider is disadvantageous.
The object of the invention is to specify, for generating the control potential for a field-effect transistor using a capacitive voltage divider, an improved circuit arrangement.
In the case of the invention, while a prescribed upper or lower filter output voltage is respectively exceeded or undershot, a current branch connected to the filter output can be activated, which loads the filter output with a current which respectively reduces or raises the filter output voltage. The current ratio between charge-reversal current and the current in the current branch connected to the filter output is dimensioned in such a way that the ratio between a current at the input or output of the filter circuit and the change in the potential at the tap of the voltage divider has approximately the same value in all operating modes of the circuit arrangement.
The current flow in the current branch connected to the filter output prevents the circuit which drives the filter from departing from its permissible output voltage range. To ensure, however, that the voltage at the tap of the voltage divider is nevertheless adjusted as if the voltage at the filter output were unlimited, part of the current, which is opposite in sign for compensation, is forwarded into the tap, such that the latter is adjusted as far as possible independently of whether or not the filter output voltage has reached a limit value. As a result, the adjustable voltage range at the tap is substantially greater than the range which corresponds to the possible variation of the filter output voltage.
The above-mentioned problem is solved in a second aspect of the invention by a circuit arrangement having the features indicated in claim 2. This circuit arrangement also serves to generate the control potential for a field-effect transistor from the output voltage of a filter circuit. The output voltage of the filter circuit is passed to a capacitive voltage divider, the tap of which carries the control potential. In addition to the control by means of the filter output voltage, the charge state of the voltage divider can be controlled for the purpose of setting a predetermined potential value relative to the reference-ground potential of the field-effect transistor by a charge-reversal current being fed in at the tap of the voltage divider, until the predetermined potential value is reached. The filter output voltage is detected and a circuit which supplies the charge-reversal current is activated in the event of the filter output voltage deviating from at least one prescribed value. In the case of the circuit according to the second aspect of the invention, the predetermined value is strictly determined relative to ground potential and/or a supply potential. In other words, the predetermined value is fixed relative to ground and/or a supply potential.
The circuits according to both aspects are combined with each other in developments of the invention.
The charge-reversal current and the current in the current branch connected to the filter output are in a prescribed current ratio which is less than 1:10, preferably approximately 1:1000. While the current in the current branch connected to the filter output should be in the microamperes range, for example, in order to prevent undesirably large charge-reversal processes at the capacitive voltage divider, a current to be fed in at the tap of a few nanoamperes suffices for correcting the control potential for the field-effect transistor in the manner described above.
The n-channel charge-reversal circuit in the development in accordance with claim 7 may contain two current branches similar to a current mirror, in which the operating path terminals of their two transistors are not connected to one another. One operating path terminal is at a first fixed potential; the other operating path terminal is connected to the filter output. As a result, the current mirror becomes active only when the potential at the filter output reaches the first fixed potential. The first current branch of the n-channel charge-reversal circuit determines the current flow in the current branch connected to the filter output. The second current branch determines the charge-reversal current at the tap of the capacitive voltage divider.
The use of the same transistor, which is at the fixed potential, for both current mirror-like current branches enables components to be saved. The prescribed current ratio between charge-reversal current and current in the current branch at the output of the filter circuit is made possible by a resistor. In order to reduce the current ratio further, the width/length ratio of the channels in the transistors of the two current mirror-like current branches may also differ.
In the embodiment according to claim 8, the charge-reversal circuit likewise has two current branches similar to a current mirror. In this case, one operating path terminal of one transistor is at a second fixed potential. The operating path terminal of the other transistor is connected to the output of the filter circuit. The circuit arrangement operates as a current mirror as soon as the potential at the output of the filter circuit reaches the second fixed potential. The charge-reversal circuit additionally contains two series-connected current mirror arrangements, the connection of which to the tap of the capacitive voltage divider enables the charge-reversal current to be fed in. In order to obtain the prescribed current ratio between charge-reversal current and current in the current branch connected to the filter output, a further resistor is used, which is arranged in the output current branch of the second current mirror and is fed with a prescribed current.